This invention relates to a solar energy concentrator, a solar energy concentrator in combination with a solar energy distributor and a solar energy concentrator in combination with a solar energy conversion chamber.
Solar energy concentrators, for use prior to converting solar energy into other useful forms of energy, are well known. Concentrated solar energy may be used in a solar furnace or converted into other forms of energy by, for example, a thermally absorbent medium, usually containing a fluid, or by a photovoltaic cell.
Known solar concentrators include parabolic dish receivers which, in order to focus direct sunlight, track the sun across the sky. These devices are usually built on towers which must be able to withstand substantial wind shear while producing a minimum of shadow on the face of the collector. The shape of the collector must remain constant over time, and tracking must be accurate to maintain an angle of incidence within one degree throughout the day. Further, when used in conjunction with a Stirling engine there are also potential problems of wind gusts creating fluctuations in the heat exchange and therefore in the power output.
Land use (8-17 acres per megawatt, 3.2-6.9 hectares/megawatt), site preparation, installation, capital costs and maintenance of heliostats in the towers are also expensive.
Solar concentrators, which use either a lens or a compound reflective surface, are also commercially available but they too require tracking mechanisms to track the sun""s movement across the sky. The cost of purchase, installation, maintenance and associated land requirements are again substantial.
Various solar concentrators are also known, which do not require tracking mechanisms, however, they suffer from various limitations, e.g., the acceptance angle of some of the concentrators are so limited that the sun""s rays can be received only for a small portion of the day. In others, a concentration factor may be satisfactory only during a limited time of day.
Moreover, known concentrators are designed for operation in direct sunlight, and do not function satisfactorily in diffuse or scattered light. For example, the solar collector of U.S. Pat. No. 4,287,880 (Geppert) comprises a reflector formed from three separate curves, which focus solar energy onto a pipe collector, such that rays of sunlight having different angles of incidence are reflected by different parts of the reflector onto the collector pipe to heat a fluid flowing, through the pipe. However, the device cannot effectively concentrate diffuse light and is therefore limited in its geographical application. The efficiency of the pipe absorber will also vary as a function of the ambient air temperature, because heat absorption, transfer and collection is external to the device.
An involute beam concentrator disclosed in U.S. Pat. No. 4,610,518 (Clegg) uses an involute chamber to convert a concentrated rectangular beam of sunlight emergent from a prismatic beam concentrator into a concentrated solar beam parallel to an axis of the concentrator. This concentrator is designed to accept input solar energy from the prismatic beam collector only over a very small range of incident angles.
The use of photovoltaic cells in the form of silicon solar cells also suffers from the disadvantage that the spectral distribution of sunlight has a maximum spectral radiance at a wavelength of 540 nanometres whereas the maximum sensitivity of the solar cells occurs at 813 nanometres. As a result, much of the energy falling on the solar cell is not converted into electricity. Moreover, these solar cells have to be cooled to maintain operational peak efficiency.
U.S. Pat. No. 4,947,292 (Vlah) discloses a lighting system designed to produce a diffuse light from a concentrated light source, in which a concentrated light source is located at the focus of a spiral-shaped horn and diffuse light is emitted from the mouth of the horn. A preferred shape of the spiral is a xe2x80x9cGolden Section spiralxe2x80x9d, also known as a volute, formed from a series of nested xe2x80x9cGolden Sectionxe2x80x9d rectangles, i.e., rectangles in which the ratio of the lengths of the larger and smaller sides is xc2xd(1+5xc2xd):1, which may be used to locate the xe2x80x9cGolden Sectionxe2x80x9d centres for the arcs which make up a xe2x80x9cGolden Sectionsxe2x80x9d spiral.
It is an object of this invention to provide a solar energy concentrator, a solar energy concentrator in combination with a solar energy distributor and a solar energy concentrator in combination with a solar energy conversion chamber which at least partially mitigate some of the difficulties of the prior art.
According to a first aspect of the invention there is provided a solar energy concentrator comprising a spiral horn having an axis perpendicular to a plane of the spiral, said concentrator including: an input aperture forming a mouth of the horn, an internal light-reflecting surface of the horn, and an exit aperture at an end of the horn remote from the mouth of the horn, said exit aperture being smaller than said input aperture and said horn continuously tapering both in the direction of said axis and in the plane of the spiral, between the input and output apertures, wherein the horn is adapted to concentrate, by multiple reflections from the internal light-reflecting surface of the horn, solar energy incident within a predetermined range of angles of incidence on the input aperture, such that concentrated solar energy is emitted from the exit aperture.
Preferably at least one director is provided in the mouth of the horn to reflect light incident from outside the predetermined range of angles of incidence into the predetermined range of angles of incidence.
Conveniently, the at least one director is a baffle disposed substantially parallel to the axis of the spiral horn.
Advantageously, the at least one director is a partial spiral horn disposed substantially perpendicular to the axis of the spiral horn in at least a portion of the spiral horn most proximate to the mouth of the horn.
Conveniently, the spiral horn has a substantially quadrilateral cross-section parallel to the axis of the horn.
Advantageously the taper in the plane of the spiral is a Golden Spiral.
Conveniently the horn is of metal.
Advantageously the metal is aluminium.
Preferably the horn has portions formed of different materials disposed along the horn spiral, the materials being adapted to withstand the temperatures reached in the respective portions of the collector in use.
Advantageously a portion of the horn proximate the exit aperture is of a ceramic material.
Conveniently the light-reflecting surface is protected by ultraviolet radiation absorbing means.
According to a second aspect of the invention there is provided a solar energy concentrator according to the first aspect in combination with distribution means in communication with the exit aperture and adapted for distributing the concentrated solar energy emitted from the exit aperture.
Preferably the distributions means includes at least one light pipe.
Advantageously the distribution means includes a diffuser for diffusing at least some of the concentrated solar energy to provide illumination.
Conveniently the diffuser is in the shape of a spiral horn.
According to a third aspect of the invention there is provided a solar energy concentrator according to said first aspect, in combination with a solar energy conversion chamber having a chamber aperture in communication with the concentrator exit aperture, the chamber containing energy conversion means for converting concentrated solar energy emitted from the exit aperture.
Advantageously, the energy conversion means includes a photovoltaic cell.
Conveniently the energy conversion means includes heat absorbing media.
Advantageously the energy conversion means includes steam generating means.
Conveniently, the energy conversion means includes a solar furnace.
Advantageously, at least some of the solar energy is reflected within the chamber before being incident on the energy conversion means.
Advantageously at least some of the solar energy undergoes wavelength changes within the chamber.
Conveniently, the solar energy undergoes wavelength increases by energy absorption and/or dissipation.
Conveniently, solar energy distribution means is provided to transmit solar energy from the exit aperture to the chamber aperture.
Advantageously the distribution means includes at least one light pipe.
The first aspect of the present invention has the advantage that the collector can collect solar energy over a large range of angles of incidence without the need for tracking mechanisms. The collector therefore efficiently collects and concentrates diffuse light. An advantage of the third aspect of the invention is that the chamber aperture of the solar energy conversion chamber approximates a black body so that most of the energy entering the chamber is absorbed within the chamber. The wavelength of solar energy may also be changed in the chamber to enable more of the energy to be absorbed by a photovoltaic cell and converted into electricity. In addition, any heat produced may also be utilised.